Many automated devices, such as answering machines, facsimile machines, modems and the like, share a standard telephone line with a subscriber's telephone set. Generally, priority should be given to use of the line by the telephone set. Problems have arisen, however, in detecting a subscriber's attempts to access the line through the telephone set. For purposes of discussion we will consider a data reporting device as one such automated system which shares a telephone line with a subscriber's telephone set.
An automated data reporting device transmits utility meter data or similar data from a subscriber's premises to a central computer database and/or receives programming and control data from the central computer. Such devices eliminate the manual labor required to go out to the subscriber's premises and read or program the meter.
Many such data reporting systems use the subscriber's telephone line and the public switched telephone network to carry the data between the subscriber's premises and the central database. For example, the data reporting device might include an autodialing modem and periodically initiate a call in to the central database to transfer the latest meter data. Some of these systems can answer calls from the central database and accept programming or control data. Data reporting devices which use the subscriber line, however, share the line with the subscriber's telephone(s) and/or any other customer premises equipment connected to the line.
In such telephone line systems, the subscriber's telephone or other equipment is the primary communication device connected to the line and must be given priority over the secondary communication device, e.g. the data reporting device. Operation of the data reporting device or other automated device connected to share the line therefore must not interfere with the subscriber's use of the telephone set on the telephone line. To prevent interference, two problems must be addressed. First, the automated device must not attempt to seize the line if the telephone is using the line at a time when the automated device is ready to access the line, e.g., to report data. Second, during use of the line by the automated device, if the subscriber picks up the telephone handset to use the telephone, the automated device must relinquish the line to the telephone.
Circuits have been proposed which detect an existing off-hook condition, and devices have been proposed to detect a pick-up of a telephone connected in parallel with the data reporting device. Such circuits, however, have tended to be rather complex, particularly if capable of both existing line use detection and pick-up detection. Due to such complexity, these circuits have been expensive to manufacture and subject to false detection results.
Detection of a pick-up of a telephone connected in parallel with the data reporting device or the like can be particularly difficult because the resultant voltage drop tends to be quite small. As a result, devices intended to detect parallel pick-up often have been ineffective.
A circuit has been proposed which would detect parallel pick-up by sensing changes in AC loading on the telephone line (U.S. Pat. No. 4,958,371 to Damoci et al). The approach in that patent, however, requires generation and application of a 300 Hz test signal to the line.
Another problem with prior art systems arises because many such systems detect line voltage directly, as an indication of the on-hook or off-hook state of the associated customer premises equipment. Different lines, however, exhibit different off-hook and on-hook voltages. For example, a residential line will typically have an on-hook voltage of 48 volts DC. The off-hook voltage typically will be as low as 3 volts DC, but the off-hook voltage may be as high as 26 volts DC. In contrast, the on-hook voltage for a PBX line typically is 22-26 volts DC. In view of this voltage overlap between off-hook of residential lines and on-hook for PBX line, it is necessary to use a different voltage detector for each different type of line.